In members such as cross members and side members for automobiles, a weight reduction has been investigated to respond to a recent trend toward a reduction in fuel consumption, and it has been attempted to increase the strength of a steel sheet from the viewpoint of ensuring the strength and collision safety of automobiles even when a thinner steel sheet is used for the members. However, since increasing the strength of the steel sheet leads to a deterioration of the formability of materials, in order to realize the weight reduction of the members, it is necessary to manufacture a steel sheet which satisfies both press formability and high strength.
Particularly, when the steel sheet is formed as structural members or reinforcing members for automobiles which have a complex shape, the steel sheet having excellent ductility is required. In recent years, a steel sheet having a tensile strength of 440 MPa class or 590 MPa class has been mainly used for frameworks of automobiles, and development of a steel sheet having a tensile strength of 980 MPa or more is desired in the future to achieve a further weight reduction.
When a steel sheet of 590 MPa class is replaced with a steel sheet of 980 MPa class, the same elongation as the elongation of the steel sheet of 590 MPa class is required in the steel sheet of 980 MPa class. Thus, development of a steel sheet which has a tensile strength of 980 MPa or more and has excellent elongation is desired.
As a steel sheet excellent in total elongation (El) in a tensile test, there is a multi-phase structure steel sheet which has a microstructure in which residual austenite as a secondary phase is dispersed in soft ferrite that is a primary phase. In the multi-phase structure steel sheet, the ductility is ensured by the ferrite and the strength is ensured by the martensitic transformation of the residual austenite, and the residual austenite is transformed into martensite at plastic working. There is a steel, which is applied the transformation, such as a transformation induced plasticity (TRIP) steel and the applications of the TRIP steel have been expanded in recent years.
Since the TRIP steel has a particularly excellent elongation compared to precipitation strengthened steel and dual phase (DP) steel (steel is consisting of ferrite and martensite), the applications of the TRIP steel is strongly desired to be expanded. Although the TRIP steel shows excellent strength and ductility, the TRIP steel has a feature of low hole expansibility in general.
Further, in order to promote a weight reduction of an automobile body in the future, a usable strength level of a high strength steel sheet should be increased as compared with that of conventional one. For example, in order to use the high strength steel sheet for a hard-to-form member such as a suspension part, formability such as hole expansibility should be improved.
In addition, when a steel sheet of 980 MPa or more is applied to the member for an automobile, in addition to properties of strength and workability, delayed fracture resistance is required. The delayed fracture is caused by stress applied to steel or hydrogen brittleness and is a phenomenon in which a structure is fractured by accumulating diffused hydrogen in a stress concentration area of the steel used as the structure.
Specifically, examples of the delayed fracture include a suddenly fractured phenomenon that a member, such as a prestressed concrete (PC) steel wire or a bolt, is suffered high stress load under the usage condition.
It is known that delayed fracture is closely related to the hydrogen which penetrates into the steel from the environment. As the hydrogen which penetrates into the steel from the environment, there are various types of hydrogen sources such as hydrogen which is contained in the atmosphere, hydrogen generated in a corrosive environment. When the hydrogen penetrates into the steel from any of the hydrogen sources, the hydrogen may induce the delayed fracture.
For this reason, as the usage environment of the steel, an environment in absence of hydrogen is desired. However, when a steel is applied to the structure or the automobile, the steel is used outdoors and the penetration of hydrogen cannot be avoided.
As the stress which acts on the steel used as the structure, a stress which is loaded on the structure and a residual stress, that some of stress generated at the forming remains inside of the steel, are included. Particularly, in the steel used as a member after forming such as a thin steel sheet for an automobile or the like, the residual stress is a significant problem compared to a thick steel plate or a steel bar (for example, a bolt) that is a product used as is with being applied no deformation. Accordingly, when a steel sheet that the delayed fracture is a problem is formed, it is desirable to form a steel sheet such that the residual stress does not remain.
For example, in Patent Document 1, there is disclosed a hot press forming method of a metal plate of which strength is increased by heating a steel sheet at a high temperature and by processing the steel sheet and then by quenching the steel sheet using a die. In this hot press forming method of a metal plate, since the steel sheet is processed at a high temperature, residual stress is alleviated by recovering dislocation which causes the residual stress and which is introduced at the processing, or by causing transformation after the processing. Therefore, very little residual stress remains in a formed product. It is possible to improve the delayed fracture resistance of the steel sheet by strengthening the steel sheet using this method. However, in this method, since it is necessary to perform heating before the pressing, the energy cost and the facility cost are high compared to cold forming. In addition, since the formed product is directly quenched at a high temperature of 600° C. or higher, the properties of the steel sheet (for example, plating properties in a plated steel sheet) are easily changed and it is difficult to control properties other than the strength and the delayed fracture resistance.
In addition, since the residual stress is present on a cutting surface in machining such as cutting or punching, there is a concern of causing delayed fracture. Thus, when a high strength steel sheet having a tensile strength of 980 MPa or more is processed, the steel sheet is cut by a method using a laser or the like which is not accompanied by direct machining, and the generation of residual stress is avoided. However, the laser cutting costs more compared to shear cutting or punching.
Therefore, it is required that the delayed fracture resistance of the steel sheet is ensured not by the forming method but by the development of materials depending on the properties required.
In product categories of a steel bar, a steel rod, and a thick steel plate, a material capable of avoiding delayed fracture by improving hydrogen embrittlement resistance has been developed. For example, in Non-Patent Document 1, there is disclosed a high strength bolt having excellent hydrogen embrittlement resistance in which fine precipitates of elements such as Cr, Mo, V and the like, which exhibit temper softening resistance, are coherently precipitated in martensite. In the high strength bolt, the steel is quenched from austenite single phase at high temperature so as to obtain a martensite single phase microstructure, and then the above fine precipitates are coherently precipitated in the martensite by tempering.
In the high strength bolt, the hydrogen penetrated into the steel is inhibited from being diffused or being concentrated on an area as a starting point of delayed fracture where stress is concentrated by using the hydrogen penetrated into the steel being trapped around the fine precipitates such as VC and the like which are coherently precipitated in the martensite. Conventionally, steel having high strength and excellent in delayed fracture resistance has been developed by utilizing such fine precipitates in the steel.
In order to improve the delayed fracture resistance by utilizing the precipitates as hydrogen trap sites such as VC and the like, it is necessary to coherently precipitate the precipitates in the martensite structure.
However, several hours or more of heat treatment is necessary to precipitate the precipitates, and there is a problem in manufacturability. That is, in a steel sheet manufactured by using general manufacturing facilities for a thin steel sheet such as continuous annealing facilities or continuous hot dip galvanizing facilities, texture control is performed in a short period of time such as several tens of minutes at most. Thus, when the thin steel sheet is manufactured, it is difficult to improve delayed fracture resistance by the precipitates.
In addition, when precipitates that are precipitated in a hot rolling process are utilized, even if the above precipitates are precipitated in the hot rolling process, an orientation relationship between the precipitates and a base structure (ferrite and martensite) is lost due to recrystallization during the subsequent cold rolling and continuous annealing. That is, in this case, the precipitates are not coherent precipitates. As a result, the delayed fracture resistance of the obtained steel sheet is significantly deteriorated.
A high strength steel sheet in which there is concern of generation of delayed fracture usually has a microstructure mainly including martensite. Although the martensite can be formed in a low temperature region, the precipitates including VC as the hydrogen trap sites in the temperature region cannot be precipitated.
As a result, when the coherent precipitates such as VC are participated in the thin steel sheet in order to improve the delayed fracture resistance, it is necessary to precipitate the precipitates by additionally performing heat treatment after the microstructure of the steel is formed by using the continuous annealing facilities or the continuous hot dip galvanizing facilities. This process brings about a significant increase in manufacturing cost.
In addition, when the above heat treatment is additionally performed on the microstructure mainly including martensite, the martensite is drastically softened. As a result, it is difficult to utilize the coherent precipitates such as VC in order to improve the delayed fracture resistance of the high strength thin steel sheet.
Here, since the steel described in Non-Patent Document 1 is steel including 0.4% or more of C and a large amount of alloy elements, the workability and the weldability which are required for the thin steel sheet are deteriorated.
In Patent Document 2, there is disclosed a thick steel plate in which hydrogen defects are reduced by oxides mainly including Ti, and Mg. However, in the thick steel plate disclosed in Patent Document 2, only the hydrogen defects that are caused by hydrogen trapped in the steel at manufacturing are reduced, and thus, hydrogen brittleness resistance (delayed fracture resistance) is not considered. Further, both the high formability and hydrogen brittleness resistance, which are required for a thin steel sheet, are not considered at all.
Conventionally, in a thin steel sheet, (1) since the sheet thickness is thin, even when hydrogen penetrates into the thin steel sheet, the hydrogen is released to the outside in a short period of time. Further, (2) since workability is prioritized, a steel sheet having a tensile strength of 900 MPa or more has not been used before. For this reason, problems of delayed fracture have been small. However, since a demand for using the high strength steel sheet as a workpiece is rapidly increasing, the development of a high strength steel sheet having excellent hydrogen brittleness resistance has been required.
As described above, the technologies for improving the hydrogen brittleness resistance that are mostly related to steel such as bolts, steel bars, and plate steel have been developed. The steel is not almost subjected to forming and is often used at proof stress or yield stress or less. Therefore, in the related art, both of the workability required for automobile members, such as cuttability or member formability (press formability), and the hydrogen embrittlement resistance after processing are not considered.
In a member after forming, a stress that is referred to as a residual stress remains the inside of the member. Although the residual stress is present in the local, the residual stress has a high value exceeding the yield stress of material in some cases. For this reason, it is required that hydrogen embrittlement not generate in the thin steel sheet under high residual stress.
Regarding the hydrogen brittleness of the thin steel sheet, for example, Non-Patent Document 2 reports the aggravation of hydrogen brittleness due to strain induced transformation of residual austenite. In Non-Patent Document 2, a formation of thin steel sheet has been considered, but an amount of the residual austenite is significantly reduced by suppressing the concentration of C in the austenite so as not to cause deterioration in the hydrogen brittleness resistance.
In addition, in the technology described in Non-Patent Document 2, since the microstructure of the high strength thin steel sheet is limited to a very narrow range, only hydrogen brittleness which is generated in a relatively short period of time is evaluated. Thus, it is difficult to fundamentally solve the problem of hydrogen brittleness when the steel sheet is actually used in a member for an automobile. Further, in the technology described in Non-Patent Document 2, the residual austenite cannot be actively utilized and the application of the steel sheet is limited.
As described above, when a large amount of residual austenite that easily occurs hydrogen brittleness is included in the steel sheet, it is very difficult to obtain a steel sheet which simultaneously demonstrates high corrosion resistance, high tensile strength, excellent delayed fracture resistance and high ductility.